1. Field of the Invention
The field of the present invention is pulsed fiber lasers, particularly pulsed laser generation in the mid-infrared range.
2. Background
Fiber lasers have several advantages, among them being high efficiency, good thermal management, and inherent robustness. Fiber lasers have been demonstrated to achieve power conversion efficiency of over 90%, and power levels of near 1 kW in continuous wave (CW) operation. For pulsed operation, however, the output pulse parameters are usually limited by the material and geometrical properties of the gain fiber. In particular, pulse energy is typically proportional to and limited by the total number of rare-earth dopant ions in the fiber core.
Different designs of pulsed fiber lasers present various advantages for different applications, with the primary advantage being the ability to control properties of the output pulse for a particular application. For example, the different rare-earth elements, such as erbium, neodymium, ytterbium, and thulium, which are often used as core dopants in fiber lasers, each provide their own advantages. Both multi-mode fibers and single-mode also present their own advantages, as do single and double clad fibers. The core size of the fiber is another parameter that may be controlled to achieve desired advantages. Examples of pulse output properties that can be controlled by selection of fiber characteristics include output wavelength band, pulse duration, pulse rate, and pulse energy, among others.
The high gain achievable with fiber lasers is advantageously used in a process of operating fiber lasers called Q-switching. The output pulse energy available in a Q-switched fiber laser can be given approximately by:E∝Vhυ(N2i−N2f),  (1)where N2i and N2f are the initial (pre-pulse, pre-switch) and final (post-pulse, post-switch) inverted ion concentrations within the fiber core per unit volume, and V is the volume of the core of the fiber. In other words, the pulse energy is proportional to the total number of ions that release energy during emission of the pulse. The higher the value of N2i, the more pulse energy can be extracted from the fiber. The highest possible value for both N2i and N2f is N, the total erbium ion concentration within the fiber core per unit volume. The value of N2f is given, in many practical cases, approximately, by Nσa/(σa+σe), where σa and σe are the absorption and emission cross-sections. The value of N2i obtained during the build-up phase in a Q-switched fiber laser is a strong function of decay time, so selecting a medium with a long decay time allows for the storage of more energy in the fiber core. Thus, an Er-doped fiber, in which the ions have a decay time of about 10 ms, is able to store more energy than a Tm-doped fiber, in which the ions have a decay time of about 0.5 ms, all other things being equal. For this reason, a Tm-doped fiber requires a significantly higher pump power to invert the medium during the build-up stage of Q-switching than does an Er-doped fiber of comparable absorption.
For a given level of dopant concentration N, the total number of active ions in a fiber is limited by the total volume of the doped core, with the result that the volume of the fiber core has a direct relationship with the total pulse energy in a Q-switched fiber laser. For example, by using a piece of commonly available single-mode Er-doped fiber, having a core radius of 1.5 μm, a length of 5 meters, and an industry standard erbium ion concentration of 0.5×1025 ions per cubic meter, in a Q-switched fiber laser, the upper limit on the output pulse energy would be on the order of only 10 μJ. In order to increase the pulse energy output, the total number of erbium ions must be increased. This can be done by increasing the length and/or the core size of the fiber to increase the overall volume of the core. For a core volume increase in the neighborhood of two orders of magnitude, the radius of the core can be increased from 1.5 microns to about 15 μm, which, if no changes are made to the core composition, renders the fiber multi-moded. Alternately, the length of the fiber can be increased from 5 meters to about 0.5 km. However, a very long fiber, due simply to the length, is impractical for most applications.
While pulsed lasers appear to offer numerous advantages for industrial, medical, and military applications, most pulsed fiber lasers remain an R&D tool, as they can not provide a performance to cost ratio which meets commercial needs. One prominent example is the pulsed Tm-doped fiber laser (TDFL). With emission spectrum from 1.8 μm to 2.1 μm, a range which contains a strong H2O absorption band, the TDFL has potential for use as a minimally invasive surgical tool. To qualify for such applications, the TDFL should generate pulse energies on the order of 10 mJ to 100 mJ, together with multi-kW peak power.